Attaboy, Austrocheirus

Paleontologists from the Argentine Museum of Natural Sciences, led by Martin Ezcurra, have published a description of a new genus of large theropod in the journal Zootaxa. Based on its ankle structure, the authors have posited Austrocheirus isaasi as a basal member of a family of mid-sized theropods called the abelisaurs. As the tyrannosaurs dominated top predator niches in the northern hemisphere in the late Cretaceous, the abelisaurs performed that role on the southern continents (with the possible exception of Tarascosaurus from France, but its abelisaurid credentials have not been satisfactorily proven).

The most famous of the abelisaurs is without a doubt Carnotaurus, instantly recognizable for its blunt snout and twin horns above the eyes. And, of course, its tiny little arms. It was the main villain in the Disney Dinosaur movie; a smaller, more realistically-sized Carnotaurus also provided brief comic relief in Jurassic Park 3. (UPDATE - Kind reader Ivan brought to my attention that my memory was faulty here - it was a Ceratosaurus in JP3, not Carnotaurus.)

Carnotaurus, via wikimedia commons.

The only abelisaur forelimbs found to date have belonged to three large members of the group: Carnotaurus, its slightly smaller Argentinian neighbor Aucasaurus, and Madagascar's Majungasaurus. Aucasaurus is remarkable for its near lack of fingers, they're so stubby and tightly packed in. Thus far, how common such reduced forelimbs and hands are among other abelisaurs is unknown, but similarly-sized Austrocheirus sheds a bit more light on the subject. Its hands aren't nearly as small, demonstrating that perhaps the reduction in forelimbs didn't necessarily correlate with greater size.

For all of the dinosaurs' fantastic features, the hands of theropods are among the most important. They're the subject of a long, complicated, and colorful debate in paleontology, and for good reason. Birds evolved from dinosaurs, after all, so understanding the process of how a theropod's hand turned into a bird's wing is of obvious value. It's important to note that the abelisaurs aren't closely related to birds. But the way the fingers are expressed in different theropods is always worth noting.

As for where Austrocheirus may fit into the grand scheme of things, this paper places it somewhere near the bottom of the abelisaur family tree, closely allied to the recently described non-abelisaur Limusaurus. One of the reasons Limusaurus, a small Chinese herbivorous theropod of the Jurassic, made a splash last year was for its unique hand format. Apropos to this new discovery, Limusaurus possessed a more reduced hand which seemed to fit in well with its abelisaur relatives. Austrocheirus muddies the waters a bit here with its relatively more robust hands; it suggests that rather than being a shared adaptation, the hands of abelisaurs and of Limusaurus were coincidentally similar. Hopefully new discoveries will clear up this tangled little knot of the theropod family tree.